Preheat oven to 200°C, gas 6. Place the sweet potato and leeks on a baking tray with 1 tbsp of oil and a big pinch of salt, mix and bake for 20-25 mins until soft.
Peel and coarsely grate the celeriac. Put the remaining oil in a large pan on medium heat. Sauté the onions and garlic, stirring, for 5 mins or until golden.
Add the celeriac and a pinch of salt, and sweat for 5 more mins, stirring, until the celeriac starts to brown. Stir in the chopped kale, cook for a couple of mins, then turn off the heat.
Place the couscous in a bowl with the tea bag and pour over 100ml boiling water. Cover and leave to stand for 5 mins. Discard the tea bag. Fluff up the grains with a fork.
Roughly chop two-thirds of the cashews. Add a pinch of chilli to the celeriac mixture, along with the tamari, salt and pepper. Fold in all the cashews, couscous and roasted vegetables. Cool.
Reduce oven to 180°C, gas 4. Line a baking tray with baking parchment. Put the pastry on the tray and spoon the filling into the centre third, lengthwise, leaving a third of the pastry either side and a little space at each end.
Form the filling into a smooth, firm mound that will hold its structure when cut. Bring the pastry round from the sides so it covers the filling. Press to seal the ends, remove any excess pastry, and fold in so it’s fully covered.
Use any pastry offcuts to create decorative leaves for the top. (optional)
Roll the welly over so the top is smooth. Add the decorative pastry leaves if using. Brush with the non-dairy milk. Bake for 30-35 minutes or until the pastry is golden.
